num = input()
if len(num) == 4:
  n1 = num [0]
  n2 = num [1]
  n3 = num [2]
  n4 = num [3]
  spis = [int(n1),int(n1+n2),int(n1+n2+n3+n4),int(n1+n2+n3),int(n2+n3),int(n2+n3+n4),int(n3+n4),int(n2),int(n3),int(n4),]
  spis.sort(reverse = 1)
  for x in spis:
    f = 1
    for d in range(2,x//2+1):
      if x % d == 0:
        f = 0
        break
    if f ==1:
      print(x)
      break
  if f == 0:
     print('0')
else: print('0')
